This invention relates to an optical fiber transmission path which exhibits single mode operation at a predetermined wavelength, although the path is comprised principally of a transmission cable which is not necessarily single mode at the predetermined wavelength.
Presently, the most frequently used wavelength for single mode operation is 1300 nm. Therefore, the commonly available single mode fibers are designed to guarantee single mode operation at 1300 nm. It is also assured that these fibers will provide single mode operation above 1300 nm, but at shorter wavelengths such as 1200 nm, they may not be single mode. In designing a single mode system, it would be useful not to exclude the lower wavelengths such as 1200 nm from single mode operation, especially for wavelength division multiplexing. The present invention provides a way to ensure single mode operation of the system at a lower wavelength such as 1200 nm, using a transmission cable that may not be single mode at that wavelength. The invention is generally applicable to other wavelength combinations as well, such as 1550 nm and 1300 nm.